The effect of providing algal food to female Crassostrea gigas broodstock sampled at a favourable time of year was investigated using pre-defined indices of oocyte quality and subsequent larval and post-larval performances. Broodstock were collected in the Aber Benoît estuary (Brittany, France) at the end of April 2002 and were divided into two groups, a starved control (S, 1 µm – filtered seawater) and a fed group (F, ad libidum algal mixture of Isochrysis galbana clone T-Iso and Chaetoceros calcitrans), maintained under these conditions for six weeks prior to fertilizations. For each conditioning type, two batches of larvae and post-larvae were reared from two identified females. Quality criteria were defined in terms of predictors (oocyte lipid contents, mature oocyte diameters, ovarian maturity and presence/absence of atresia) and validators (larval growth, algal consumption and time to settlement, and post-larval growth). Each female was considered as a treatment, and parametric and non-parametric ANOVAs were performed, where appropriate, on predictors and validators between each treatment. In both conditions, a surplus of oocytes was produced, in relation to expected market conditions. No effect of the conditioning treatment was detected either on oocyte quality predictors or on subsequent validators. It is likely that winter reserve accumulation is more important than trophic conditioning for successful gametogenesis in the productive Aber Benoît environment. Given the considerable cost of abundant feeding during the conditioning of broodstock collected at the end of winter and destined for gamete stripping (the prevalent practice in France), we suggest that feeding be minimal or suppressed altogether when the condition index is favourable. L'effet de l'apport de nourriture algale à des géniteurs femelles de l'huître Crassostrea gigas échantillonnés à une période favorable de l'année a été étudié, utilisant des indices prédéfinis de qualité ovocytaire et de performances larvaires et post-larvaires subséquentes. Les géniteurs ont été collectés dans l'Aber Benoît (Bretagne, France) à la fin avril 2002 et ont été divisés en deux groupes, un groupe témoin non nourri (S, eau de mer filtrée à 1 µm) et un groupe nourri (F, mélange algal ad libidum de Isochrysis galbana clone T-Iso et Chaetoceros calcitrans), maintenus dans ces conditions durant six semaines avant les fécondations. Pour chaque type de conditionnement, deux lots de larves et de post-larves issus de deux femelles distinctes ont été élevés. Les critères de qualité ont été définis en termes de prédicteurs (teneurs en lipides des ovocytes, diamètres des ovocytes mûrs, maturité ovarienne et présence/absence d'atrésie) et de validateurs (croissance larvaire, consommation algale et temps à la métamorphose, puis croissance post-larvaire). Chaque femelle a été considérée comme un traitement, et des analyses de variance, ANOVA, paramétriques et non-paramétriques ont été effectuées, selon les cas, sur les prédicteurs et les validateurs entre chaque traitement. Pour les deux types de conditionnement, un surplus d'ovocytes a été produit, par rapport à la demande potentielle du marché. Aucun effet du régime de conditionnement n'a été détecté, ni sur les prédicteurs de qualité ovocytaire, ni sur les validateurs subséquents. Il semblerait que pour une gamétogenèse réussie, l'accumulation de réserves pendant l'hiver soit plus importante que le conditionnement trophique, dans l'environnement productif de l'Aber Benoît. Etant donné le coût considérable d'une alimentation abondante pendant le conditionnement de géniteurs prélevés à la fin de l'hiver et destinés au « stripping » (la pratique courante en France), nous proposons de réduire, voire même de supprimer l'alimentation pendant le conditionnement lorsque l'indice de condition est favorable.

Understanding the quality cues which govern the grazing of phytoplankton by suspension feeders is essential to understanding seston dynamics and benthic–pelagic coupling. We studied the effect of functioning cellular status on selection of two common coastal diatoms, Coscinodiscus perforatus var. pavillardii and Actinoptychus senarius, grazed upon by the commercially farmed oyster, Crassostrea gigas Thunberg. Two contexts were investigated, corresponding to those naturally encountered by oysters and their prey: (i) diatoms (C. perforatus var. pavillardii) larger than the size allowing selection on the gill (hence subject to selection on the labial palps only) and (ii) diatoms (A. senarius) within the size range allowing selection on the gill (hence subject to selection on both the gills and labial palps). Both intact and naturally dead cells possessed a perifrustular envelope, but the naturally dead cells lacked intracellular organic content. Oysters were able to discriminate between intact and naturally dead (empty) cells, both in the absence of prior selection on the gill and subsequent to prior selection on the gill. In contrast to previous findings in scallops, functioning cellular status is thus an important determinant of diatom selection and ingestion by oysters. This fine degree of discrimination may have evolved as an adaptation to the high turbidity, low food-quality estuarine environment typically colonized by oysters. The reduced organic content of rejected diatom cells in oyster pseudofaeces may affect the organic content of biodeposits, and subsequently the dynamics of nutrient release to the water column during mixing and resuspension.

Economic exploitation is one means to offset the cost of controlling invasive species, such as the introduced Pacific oyster (Crassostrea gigas Thunberg) on the French Atlantic coast. Total lipid and phospholipid (PL) fatty acids (FAs) and sterols were examined in an invasive population of C. gigas in Bourgneuf Bay, France, over four successive seasons, with a view to identify possible sources of exploitable substances. The total lipid level (% dry weight) varied from 7.1% (winter) to 8.6% (spring). Of this, PLs accounted for 28.1% (spring) to 50.4% (winter). Phosphatidylcholine was the dominant PL throughout the year (up to 74% of total PLs in winter). Plasmalogens were identified throughout the year as a series of eleven dimethylacetals (DMAs) with chain lengths between C₁₆ and C₂₀ (up to 14.5% of PL FAs + DMAs in winter). Thirty-seven FAs were identified in the PL FAs. Eicosapentaenoic acid (20:5n-3 EPA/7.53% to 14.5%) and docosahexaenoic acid (22:6n-3 DHA/5.51% to 9.5%) were the dominant polyunsaturated FAs in all seasons. Two non-methylene-interrupted dienoic (NMID) FAs were identified in all seasons: 7,13-docosadienoic and 7,15-docosadienoic acids, the latter being present at relatively high levels (up to 9.6% in winter). Twenty free sterols were identified, including cholesterol at 29.9% of the sterol mixture and about 33% of phytosterols. C. gigas tissues thus contained exploitable lipids for health benefits or as a potential source of high-quality commercial lecithin.